Effect of maize row orientation configurations on the photosynthetic characteristics, leaf structure and yield of soybean in relay strip intercropping systems

A maize-soybean intercropping system is conducive to increasing land use, expanding soybean production areas, and improving soybean quality. Furthermore, the field configuration directly affects soybean growth and yield formation. This study analyzed the effects of varying row direction configurations on the morphology, photosynthetic physiological parameters, and yield of soybeans in relay strip intercropping systems to determine the optimal row direction for maize-soybean relay strip intercropping systems in southern China. A single-factor randomized block design was used in this study. The intercropping patterns used wide-narrow-row planting with alternating strips of maize and soybeans. The ratio of maize and soybean rows per strip in the relay strip intercropping systems was 2∶2. There were six treatments: monoculture maize (CKm) and soybean (CKs) both with northwest-southeast row direction, and the east-west, north-south, northwest-southeast, and northeast-southwest row direction of the relay strip intercropping systems of soybean−maize. The determination indices included the light environment, plant height, leaf area index, photosynthetic pigment, photosynthetic parameters, leaf structure, grain to leaf ratio, and yield. The results showed that the shading degrees of intercropped soybeans in the east-west and northwest-southeast directions were significantly lower than that in the north-south and northeast-southwest directions (P<0.05). Soybeans planted in the east-west direction received maximum light. The leaf area index (LAI), aboveground biomass, photosynthetic pigment content, net photosynthetic rate, leaf productivity, leaf thickness, and stomatal density of soybeans in the strip intercropping treatments decreased significantly, with the overall trend of the east-west direction treatment > northwest-southeast direction treatment > north-south direction treatment > northeast-southwest direction treatment. The LAI, aboveground biomass, photosynthetic pigment content, net photosynthetic rate, thickness, stomatal density, stomatal length, and stomatal opening of the east-west direction treatment were significantly higher than those of the other relay strip intercropping treatments at each growth stage. In terms of yield and benefit, under the east-west direction treatment, soybean yield, grain-leaf ratio, grain leaf ratio at 1–5 nodes, and grain leaf ratio at 3–4 nodes were 1932.66 kg∙hm⁻², 1074.25 g∙m⁻², 498.50 g∙m⁻², and 207.59 g∙m⁻², respectively, which were higher than those in other treatments (P<0.05). Compared with the CKm treatment, the actual yield of maize in the north-south, northwest-southeast, and northeast-southwest direction treatments decreased significantly, whereas that in the east-west direction treatment increased by 1.67%. Moreover, the east-west direction treatment had the highest yield contribution rate (79.44%) of maize, and the land equivalent ratio was 1.66. In southern China, planting in the east-west row direction can better demonstrate the advantages of maize-soybean relay strip intercropping systems.